"We need a big signal before we can tell unambiguously that the ozone layer ... is recovering," Molina told reporters ahead of the 16th Meeting of the Parties to the Montreal Protocol on Substances that Deplete the Ozone Layer, an annual event where politicians and scientists make adjustments to a global treaty requiring nations to stop using chemicals that destroy the ozone layer.

The 1987 protocol established a system by which nations would gradually eliminate chemicals that destroy the ozone layer, which prevents ultraviolet light from reaching the earth and harming living things.

The treaty, signed by nearly 200 countries, is considered the most successful international environmental treaty.

One of the major chemicals still to be phased out was methyl bromide, "a significant contributor" to the ozone layer destruction, Molina said.

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Under the protocol, wealthy nations are to eliminate production of methyl bromide completely by Jan 1, 2005. However, countries that say they have a critical need for methyl bromide and have no alternatives can seek exemptions.

Delegates to the five-day meeting, which begins in Prague on Monday, have to decide on exemptions for 2006. The United States and 12 other countries are to seek exemptions for that year.

The U.S government has argued in the past that exemptions from the treaty were needed to avoid disrupting agricultural markets.

At the last year's meeting in Nairobi, Kenya, the U.S. delegation asked to increase by more than 8 percent production of the fumigant used to control insects, nematodes, weeds and pathogens.

Negotiators from the European Union and poor countries at that meeting called the proposed increase was too high and refused to agree to the exemption.

In April, Republicans in the House of Representatives promoted a bill that would let the United States ignore the treaty and allow U.S. production of methyl bromide to continue even if other countries don't agree.